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NATS 101 - 34Lecture 2
Hurricane Dean & 2006 climate anomalies
Atmospheric Composition
Density, Pressure & Temperature
http://www.ncdc.noaa.gov/oa/climate/research/2006/ann/ann06.html
Atmospheric CompositionPermanent Gases
• N2 and O2 are most abundant gases
• Percentages hold constant up to 80 km
• Ar, Ne, He, and Xe are chemically inert
• N2 and O2 are chemically active, removed & returned
Ahrens, Table 1.1, 4th Ed.
Atmospheric CompositionImportant Trace Gases
Ahrens, Table 1.1, 3rd ed.
Which of these is now wrong even in the 4th edition of Ahrens?
CO2 Trend
“Keeling Curve”
Some gases vary by season and over many years.
The CO2 trend is the cause for concern about global warming.
CO2 increases in northern spring,
decreases in northern fall
See http://earthguide.ucsd.edu/globalchange/keeling_curve/01.html
H2O Vapor VariabilityPrecipitable Water (mm)
Some gases can vary spatially and daily
Two Important Concepts
Let’s introduce two new concepts...
Density
Pressure
What is Density?
Density () = Mass (M) per unit Volume (V)
= M/V
= Greek letter “rho”
Typical Units: kg/m3, gm/cm3
Mass =
# molecules (mole) molecular mass (gm/mole)
Avogadro number (6.023x1023 molecules/mole)
Density Change
Density () changes by altering eithera) # molecules in a constant volumeb) volume occupied by the same # molecules
ab
What is Pressure?
Pressure (p) = Force (F) per unit Area (A)
Typical Units: pounds per square inch (psi), millibars (mb),
inches Hg
Average pressure at sea-level:
14.7 psi
1013 mb
29.92 in. Hg
PressureCan be thought of as weight of air above you.
(Note that pressure acts in all directions!)
So as elevation increases, pressure decreases.
Higher elevation Less air aboveLower pressure
Lower elevation More air above Higher pressureBottom
Top
Density and Pressure Variation
Key Points
1. Both decrease rapidly with height
2. Air is compressible, i.e. its density varies
Ahrens, Fig. 1.5
Why rapid change with height?
Consider a spring with 10 kg bricks on top of it
The spring compresses a little more with each addition of a brick. The spring is compressiblecompressible.
10 kg 10 kg
10 kg
10 kg
10 kg
10 kg
Why rapid change with height?
Now consider several 10 kg springs piled on top of each other.
Topmost spring compresses the least!
Bottom spring compresses the most!
The total mass above you decreases rapidly w/height.
massmass
massmass
massmass
massmass
Why rapid change with height?
Finally, consider piled-up parcels of air, each with the same # molecules.
The bottom parcel is squished the most.
Its density is the highest.
Density decreases most rapidly at bottom.
Why rapid change with height?
Each parcel has the same mass (i.e. same number of molecules), so the height of a parcel represents the same change in pressure p.
Thus, pressure must decrease most rapidly near the bottom.
pp
pp
pp
pp
A Thinning Atmosphere
Bottom
Top Lower density, Gradual drop
Higher densityRapid decrease
NASA photo gallery
Pressure Decreases Exponentially with Height
Logarithmic Decrease• For each 16 km
increase in altitude, pressure drops by factor of 10.
48 km - 1 mb 32 km - 10 mb 16 km - 100 mb 0 km - 1000 mb
100 mb
10 mb
1 mb
16 km
32 km
48 km
Ahrens, Fig. 1.5
Water versus Air
Pressure variation in water acts more like bricks, close to incompressible, instead of like springs.
Air:Lower density, Gradual drop
Higher densityRapid decrease Bottom
Top
Bottom
Top Water:Constant drop
Constant drop
Equation for Pressure Variation
We can Quantify Pressure Change with Height /(16km)
MSL
MSL
where
is elevation in kilometers (km)
is pressure in millibars (mb)
at elevation z in meters (km)
is pre
(at elevation zin km)
ssure (mb
1
) at mean sea l
0
leve
Z
z
p
p
p
p −= ×
What is Pressure at 2.8 km?(Summit of Mt. Lemmon)
Use Equation for Pressure Change/(16 km)
MSL
(2.8km) /(16 km)
0.175
MSL
(at elevation Zin km) 10
(2.8 km) 1013mb 10
(2.8 km) 1013mb
set = 2.8 km, 10
10
(2.8 km) 1013mb 0.668 677mb
13 mb
Zp p
p
p
p
Z
p
−
−
−
= ×
= ×
= ×= × =
=
What is Pressure at Tucson?
Use Equation for Pressure Change
Let’s get cocky…
How about Denver? Z=1,600 m
How about Mt. Everest? Z=8,700 m
You try these examples at home for practice
/(16km)M
MS
S
L
L(at e
set =
levation Zin
800 m
km) 10
, 1013 mb
Z
Z p
p p −= ×=
Temperature (T) Profile• More complex than
pressure or density • Layers based on
the Environmental Lapse Rate (ELR), the rate at which temperature decreases with height.
inversion
isothermal
6.5oC/km
Ahrens, Fig. 1.7
Higher AtmosphereMolecular Composition• Homosphere- gases
are well mixed. Below 80 km. Emphasis of Course.
• Heterosphere- gases separate by molecular weight, with heaviest near bottom. Lighter gases (H, He) escape.
Ahrens, Fig. 1.8
Summary• Many gases make up air
N2 and O2 account for ~99%
Trace gases: CO2, H2O, O3, etc.Some are very important…more later
• Pressure and Density Decrease rapidly with height
• TemperatureComplex vertical structure
Reading Assignment
• Ahrens
Pages 13-22; 427-428 (Appendix C)
Problems 1.17, 1.18, 1.20
(1.17 Chapter 1, Question 17)
Don’t Forget the 4”x6” Index Cards
